Jürgen Bruckner

A Comparative Study of Microstructure and Residual Stresses of CMT-, MIG- and Laser-Hybrid Welds

Recently a new welding technique, the so-called ‘Cold Metal Transfer’ (CMT) technique was introduced, which due to integrated wire feeding leads to lower heat input and higher productivity compared to other gas metal arc (GMA) technique. Here microstructure formation and residual stress state in aluminum CMT welds are characterized and compared to those produced by pulsed MIG- and Laser-hybrid techniques. The results show a small heat affected zone (HAZ) in the MIG weld, the HAZ in the CMT and the laser hybrid welds was not visible by optical and scanning electron microscopy. Compared to the MIG welding the CMT process appears to introduce slightly smaller maximum tensile residual stresses into the weld.

Study of Microstructure and Residual Stresses in Dissimilar Al/Steels Welds Produced by Cold Metal Transfer

Recently a new welding technique, the so-called ‘Cold Metal Transfer’ (CMT) technique was introduced, which due to integrated wire feeding leads to lower heat input and higher productivity compared to other gas metal arc (GMA) techniques. Here microstructure formation and residual stress state in dissimilar steel to aluminum CMT welds are investigated. The intermetallic phase seam between the filler and the steel is only a few micrometers thick. Residual stress analyses reveal the formation of the typical residual stress state of a weld without phase transformation. Both in longitudinal and in transversal direction compressive residual stresses exist in the steel plate parent material, tensile residual stresses are present in the heat affected zone of the steel and the aluminum alloy. The area containing tensile residual stresses is larger in the aluminum alloy due to its higher heat conductivity than in the steel. Due to the symmetry in the patented voestalpine welding geometry and the welding from bottom and face side of the weld, the residual stress distributions at the top and at the bottom side of the weld are very similar.

Metallurgical approach for the development of a hot crack-resistant metal-cored wire

During the last years, the automotive industry has striven to decrease the emission rates by raising combustion temperatures in the engine. As the exhaust temperature increases, this places higher demands on the exhaust system components and the filler metals used for welding. Due to the ever increasing requirements placed on welding efficiency, the use of metal-cored stainless steel wires continues to grow. High quality can be achieved at high welding speeds with minimum amount of rework, when these wires are used. As hot cracking can occur when welding high-temperature stainless steel grades, the weldability is restricted by the resistance to solidification cracking. Within the scope of this paper, two types of metal-cored wires with significant difference in δ-ferrite and manganese contents were compared to each other concerning hot crack susceptibility. Programmable Deformation Cracking and Modified Varestraint Transvarestraint tests were performed to determine the hot cracking liability. The wire with the higher amount of δ-ferrite showed better resistance than the wire with higher manganese content. Good weldability at high welding speed was confirmed by robotic welding. Additionally, the influence of the high temperature on the formation of σ-phase and its effect on the impact toughness was investigated.